- Quantum chemistry
**Quantum chemistry**is a branch oftheoretical chemistry , which appliesquantum mechanics andquantum field theory to address issues and problems inchemistry . The description of the electronic behavior ofatom s andmolecule s as pertaining to theirreactivity is one of the applications of quantum chemistry. Quantum chemistry lies on the border betweenchemistry andphysics , and significant contributions have been made by scientists from both fields. It has a strong and active overlap with the field ofatomic physics andmolecular physics , as well asphysical chemistry .Quantum chemistry is that branch of physical chemistry,which applies quantum mechanics and field theory to understand electronic behaviour of atoms and molecules,and their reactivity.Quantum chemistry mathematically describes the fundamental behavior of

matter at the molecular scale. [*cite web | title = Quantum Chemistry | url = http://cmm.cit.nih.gov/modeling/guide_documents/quantum_mechanics_document.html | publisher =*] It is, in principle, possible to describe all chemical systems using this theory. In practice, only the simplest chemical systems may realistically be investigated in purely quantum mechanical terms, and approximations must be made for most practical purposes (e.g.,National Institutes of Health | work = The NIH Guide to Molecular Modeling | accessdate = 2007-09-08Hartree-Fock ,post Hartree-Fock orDensity functional theory , seecomputational chemistry for more details). Hence a detailed understanding ofquantum mechanics is not necessary for most chemistry, as the important implications of the theory (principally the orbital approximation) can be understood and applied in simpler terms.In quantum mechanics (several applications in computational chemistry and quantum chemistry), the Hamiltonian, or the physical state, of a particle can be expressed as the sum of two operators, one corresponding to

kinetic energy and the other topotential energy . TheHamiltonian in theSchrödinger wave equation used in quantum chemistry does not contain terms for the spin of the electron.Solutions of the Schrödinger equation for the hydrogen atom gives the form of the wave function for

atomic orbital s, and the relative energy of the various orbitals. The orbital approximation can be used to understand the other atoms e.g.helium ,lithium andcarbon .**History**The

**history of quantum chemistry**essentially began with the 1838 discovery ofcathode rays byMichael Faraday , the 1859 statement of theblack body radiation problem byGustav Kirchhoff , the 1877 suggestion byLudwig Boltzmann that the energy states of a physical system could be discrete, and the 1900 quantum hypothesis byMax Planck that any energy radiating atomic system can theoretically be divided into a number of discrete energy elements "ε" such that each of these energy elements is proportional to thefrequency "ν" with which they each individually radiateenergy , as defined by the following formula: :$epsilon\; =\; h\; u\; ,$where "h" is a numerical value called

Planck’s Constant . Then, in 1905, to explain thephotoelectric effect (1839), i.e., that shining light on certain materials can function to eject electrons from the material,Albert Einstein postulated, based on Planck’s quantum hypothesis, thatlight itself consists of individual quantum particles, which later came to be calledphotons (1926). In the years to follow, this theoretical basis slowly began to be applied to chemical structure, reactivity, and bonding.**Electronic structure**The first step in solving a quantum chemical problem is usually solving the

Schrödinger equation (orDirac equation inrelativistic quantum chemistry ) with theelectronic molecular Hamiltonian . This is called determining the**electronic structure**of the molecule. It can be said that the electronic structure of a molecule or crystal implies essentially its chemical properties.**Wave model**The foundation of quantum mechanics and quantum chemistry is the

**wave model**, in which the atom is a small, dense, positively charged nucleus surrounded by electrons. Unlike the earlierBohr model of the atom, however, the wave model describes electrons as "clouds" moving in orbitals, and their positions are represented by probability distributions rather than discrete points. The strength of this model lies in itspredictive power . Specifically, it predicts the pattern of chemically similar elements found in the periodic table. The wave model is so named because electrons exhibit properties (such as interference) traditionally associated with waves. Seewave-particle duality .**Valence bond**:main|Valence bond theory

Although the mathematical basis of quantum chemistry had been laid by Schrödinger in 1926, it is generally accepted that the first true calculation in quantum chemistry was that of the German physicists

Walter Heitler andFritz London on the hydrogen (H_{2}) molecule in 1927. Heitler and London's method was extended by the American theoretical physicistJohn C. Slater and the American theoretical chemistLinus Pauling to become the**Valence-Bond (VB)**[or**Heitler-London-Slater-Pauling (HLSP)**] method. In this method, attention is primarily devoted to the pairwise interactions between atoms, and this method therefore correlates closely with classical chemists' drawings of bonds.**Molecular orbital**:main|Molecular orbital theory

An alternative approach was developed in 1929 by

Friedrich Hund andRobert S. Mulliken , in whichelectron s are described by mathematical functions delocalized over an entiremolecule . The**Hund-Mulliken**approach or**molecular orbital (MO) method**is less intuitive to chemists, but has turned out capable of predicting spectroscopic properties better than the VB method. This approach is the conceptional basis of theand furtherHartree-Fock methodpost Hartree-Fock methods.**Density functional theory**:main|Density functional theory

The

**Thomas-Fermi model**was developed independently by Thomas and Fermi in 1927. This was the first attempt to describe many-electron systems on the basis ofelectronic density instead ofwave function s, although it was not very successful in the treatment of entire molecules. The method did provide the basis for what is now known as**density functional theory**. Though this method is less developed than post Hartree-Fock methods, its lower computational requirements allow it to tackle largerpolyatomic molecule s and evenmacromolecule s, which has made it the most used method incomputational chemistry at present.**Chemical dynamics**A further step can consist of solving the

Schrödinger equation with the totalmolecular Hamiltonian in order to study the motion of molecules. Direct solution of the Schrödinger equation is called "quantum molecular dynamics", within thesemiclassical approximation "semiclassical molecular dynamics", and within theclassical mechanics framework "molecular dynamics (MD)". Statistical approaches, using for exampleMonte Carlo method s, are also possible.**Adiabatic chemical dynamics**:"Main article: Adiabatic formalism or Born-Oppenheimer approximation"In

**adiabatic dynamics**, interatomic interactions are represented by single scalarpotential s calledpotential energy surface s. This is theBorn-Oppenheimer approximation introduced by Born and Oppenheimer in 1927. Pioneering applications of this in chemistry were performed by Rice and Ramsperger in 1927 and Kassel in 1928, and generalized into theRRKM theory in 1952 by Marcus who took thetransition state theory developed by Eyring in 1935 into account. These methods enable simple estimates of unimolecularreaction rates from a few characteristics of the potential surface.**Non-adiabatic chemical dynamics**:main|Vibronic coupling

**Non-adiabatic dynamics**consists of taking the interaction between several coupled potential energy surface (corresponding to different electronicquantum state s of the molecule). The coupling terms are called**vibronic couplings**. The pioneering work in this field was done by Stueckelberg, Landau, and Zener in the 1930s, in their work on what is now known as theLandau-Zener transition . Their formula allows the transition probability between twodiabatic potential curves in the neighborhood of anavoided crossing to be calculated.**Quantum chemistry and quantum field theory**The application of

quantum field theory (QFT) to chemical systems and theories has become increasingly common in the modern physical sciences. One of the first and most fundamentally explicit appearances of this is seen in the theory of thephotomagneton . In this system, plasmas, which are ubiquitous in both physics and chemistry, are studied in order to determine the basic quantization of the underlying bosonic field. However, quantum field theory is of interest in many fields of chemistry, including:nuclear chemistry ,astrochemistry ,sonochemistry , andquantum hydrodynamics . Field theoretic methods have also been critical in developing the ab initio Effective Hamiltonian theory of semi-empirical pi-electron methods.**See also***

Quantum chemistry computer programs

*Computational chemistry

*Theoretical chemistry

*Physical chemistry

*Atomic physics

*Theoretical physics

*Condensed matter physics

*International Academy of Quantum Molecular Science

*Quantum electrochemistry **Further reading***cite book |author=Pauling, L.|title=

General Chemistry |publisher=Dover Publications|year=1954|id= ISBN 0-486-65622-5

*Pauling, L., and Wilson, E. B. "Introduction to Quantum Mechanics with Applications to Chemistry" (Dover Publications) ISBN 0-486-64871-0

*Atkins, P.W. "Physical Chemistry" (Oxford University Press) ISBN 0-19-879285-9

* McWeeny, R. "Coulson's Valence" (Oxford Science Publications) ISBN 0-19-855144-4

*Landau, L.D. and Lifshitz, E.M. "Quantum Mechanics:Non-relativistic Theory"(Course of Theoretical Physics vol.3) (Pergamon Press)

*Eric R. Scerri, The Periodic Table: Its Story and Its Significance, Oxford University Press, 2006. Considers the extent to which chemistry and especially the periodic system has been reduced to quantum mechanics. ISBN 0-19-530573-6**External links*** [

*http://vergil.chemistry.gatech.edu/notes/index.html The Sherrill Group - Notes*]

* [*http://www.shodor.org/chemviz/ ChemViz Curriculum Support Resources*]

* [*http://www.quantum-chemistry-history.com/ Early ideas in the history of quantum chemistry*]**Nobel lectures by quantum chemists*** [

*http://nobelprize.org/chemistry/laureates/1998/kohn-lecture.html Walter Kohn's Nobel lecture*]

* [*http://nobelprize.org/chemistry/laureates/1992/marcus-lecture.html Rudolph Marcus' Nobel lecture*]

* [*http://nobelprize.org/chemistry/laureates/1966/mulliken-lecture.html Robert Mulliken's Nobel lecture*]

* [*http://nobelprize.org/chemistry/laureates/1954/pauling-lecture.html Linus Pauling's Nobel lecture*]

* [*http://nobelprize.org/chemistry/laureates/1998/pople-lecture.html John Pople's Nobel lecture*]**References**

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